1. Field of the Invention
The present invention relates to an optical device such as a digital camera and, more particularly, to a structure which suppresses an alien substance such as dust from adhering to the surface of an optical member disposed on or near the focal plane, e.g., a solid image sensing device, optical filter, or lens incorporated in an optical device.
2. Description of the Related Art
Conventionally, if an alien substance such as dust adheres to the vicinity of the focal plane of the photographing lens of a digital single-lens reflex camera of lens interchangeable type, the shadow of the alien substance is sensed by a solid image sensing device. Such an alien substance is supposed to be external dust which enters upon lens replacement, or fine abrasion powder of resin or the like of the structural component of the shutter or mirror, which is generated as the component operates in the camera. An alien substance generated due to these reasons enters especially between the protective cover glass of the solid image sensing device and an infrared cut filter or optical low-pass filter (to be abbreviated as an LPF hereinafter) arranged on the entire surface of the cover glass. To remove the alien substance, it is necessary to disassemble the camera. Hence, a sealed structure is very effective for preventing any alien substance from entering between the optical filter and the cover glass of the solid image sensing device.
However, if an alien substance adheres to the surface of the optical filter on the opposite side of the solid image sensing device, and its position is near the focal plane, the shadow of the alien substance is sensed by the solid image sensing device.
To solve this problem, the cover glass surface of the solid image sensing device is cleaned by a wiper (Japanese Patent Laid-Open No. 2003-005254 (FIGS. 1 and 9 on page 8)). With this structure, any alien substance adhering to the cover glass surface of the solid image sensing device or the outermost surface (e.g., the optical filter surface) of the dustproof structure can be removed without detaching the lens or disassembling the camera. In the arrangement disclosed in Japanese Patent Laid-Open No. 2003-005254, however, the cover glass surface of the solid image sensing device or the outermost surface of the dustproof structure is rubbed by the wiper. If the alien substance is hard, like metal powder, it can make scratches on the cover glass surface of the solid image sensing device or the outermost surface of the dustproof structure. In addition, the alien substance temporarily removed by the wiper floats in the camera and re-adheres to the cover glass surface of the solid image sensing device or the outermost surface of the dustproof structure.
As a solution to the former problem, a transparent electrode is formed on each of the cover glass surface of the solid image sensing device and the surface of the optical filter to suppress any alien substance from adhering to the cover glass surface of the solid image sensing device (Japanese Patent Laid-Open No. 2000-029132 (FIG. 2 on page 8)). According to the technique described in Japanese Patent Laid-Open No. 2000-029132, potentials are applied to the transparent electrodes provided on the cover glass surface of the solid image sensing device and the surface of the optical filter to neutralize static electricity generated on the cover glass surface of the solid image sensing device and the surface of the optical filter. This structure can suppress dust from adhering to the cover glass surface of the solid image sensing device and the surface of the optical filter due to static electricity. However, since the transparent electrodes are provided on the cover glass surface and the surface of the optical filter, the light transmittance to the image sensing device decreases, resulting in an adverse optical effect. In addition, since static electricity generated on the cover glass surface of the solid image sensing device and the surface of the optical filter changes depending on the environment (temperature and humidity) or use conditions, control to neutralize the static electricity is difficult. If the static electricity cannot be neutralized, the suppressing effect is insufficient.
As another technique, a fine periodic structure is formed on the surface of a silicon plate by a femtosecond (10−15 sec) laser to weaken the adhesive force of fine particles in the atmosphere to the plate (The Society of Powder Technology, Japan, Proceedings of Autumn Annual Meeting in 2004 (FIGS. 1 and 2 on page 34 and FIG. 4 on page 35)). As is reported, the van der Waals force and crosslinking force of liquid, which act between glass beads and the silicon plate, is decreased by the fine periodic structure. However, this reference contains no mention of whether the fine periodic structure can be formed on the cover glass of an optical element or the surface of an optical filter by the femtosecond laser. The period of the fine periodic structure is about 620 nm, which falls within the visible light range. Since an adverse optical effect may occur, the technique of this report cannot directly be applied as a measure against an alien substance adhering to the cover glass surface of the solid image sensing device or the surface of the optical filter.